Theoretical infrared signature of OH defects in Fe3+, Cr3+ and Al3+-doped enstatite

The infrared spectroscopic properties of selected defects involving one proton and one nearby M³⁺ (M = Al, Cr, Fe) substitution in orthoenstatite are investigated by first-principles calculations. Based on the theoretical results, the absorption bands experimentally observed on synthetic samples with high crystalline quality and low doping levels can be assigned to specific defect configurations. Most of them correspond to Mg vacancies at M2 sites locally compensated by one proton and one M³⁺ cation at a nearby M1 site. This confirms that the M³⁺ + H⁺ = 2 Mg²⁺ exchange mechanism is the dominant hydrogen incorporation mechanism at the lowest concentration levels in doped enstatite. At higher concentration levels, more complex incorporation mechanisms could become dominant in Al-bearing samples.